Molecular and morphological configuration for 2-Arachidonoylglycerol-mediated retrograde signaling at mossy cell-granule cell synapses in the dentate gyrus

2-Arachidonoylglycerol (2-AG) is the endocannabinoid that mediates retrograde suppression of neurotransmission in the brain. In the present study, we investigated the 2-AG signaling system at mossy cell (MC)-granule cell (GC) synapses in the mouse dentate gyrus, an excitatory recurrent circuit where endocannabinoids are thought to suppress epileptogenesis. First, we showed by electrophysiology that 2-AG produced by diacylglycerol lipase α(DGLα) mediated both depolarization-induced suppression of excitation and its enhancement by group I metabotropic glutamate receptor activation at MC-GC synapses, as they were abolished in DGLα-knock-out mice. Immunohistochemistry revealed that DGLα was enriched in the neck portion of GC spines forming synapses with MC terminals, whereas cannabinoid CB₁ receptors accumulated in the terminal portion of MC axons. On the other hand, the major 2-AG-degrading enzyme, monoacylglycerol lipase (MGL), was absent at MC-GC synapses but was expressed in astrocytes and some inhibitory terminals. Serial electron microscopy clarified that a given GC spine was innervated by a single MC terminal and also contacted nonsynaptically by other MCterminals making synapses with otherGCspines in the neighborhood. MGL-expressing elements, however, poorly coveredGCspines, amounting to 17% of the total surface of GC spines by astrocytes and 4% by inhibitory terminals. Our findings provide a basis for 2-AG-mediated retrograde suppression of MC-GC synaptic transmission and also suggest that 2-AG released from activated GC spines is readily accessible to nearby MC-GC synapses by escaping from enzymatic degradation. This molecular-anatomical configuration will contribute to adjust network activity in the dentate gyrus after enhanced excitation.